Field of the Invention
The present invention provides an enamelling steel sheet less susceptible to occurrence of fish-scales. The term "enamelling steel sheet" used in the present invention represents a steel sheet material prior to the application of enamel and includes both hot rolled and cold rolled steel sheets.
It is well known when the enamelling steel sheet is enamelled, various surface defects, such as fish-scales, foams and firing strains often appear. The fish-scales in particular appear during a period of several days to several months after the firing of the enamel, and once they appear, they are very hard, if not at all impossible, to eliminate. Much money and labour are required for the remedy.
Therefore, strong demands have been made for an enamelling steel sheet free from occurrence of the fish-scales.
It has been generally accepted that the fish-scales are caused by a large amount of atomic hydrogen being absorbed into the steel sheet from the enamel glaze and the moisture within the firing furnace during the firing of the enamel at high temperatures. Thus, as the temperature of the enamelled steel sheet is lowered, the solubility of the hydrogen in the steel sheet decreases and the hydrogen collects together in the interlayer between the enamel layer and the steel sheet in the form of molecular hydrogen under a high pressure, thus breaking the enamel film formed by the firing process and exploding out of the enamelled steel sheet to cause the fish-scales on the sheet surface.
Therefore, in order to prevent the occurrence of fish-scales, it will be necessary to decrease the moisture which is the source of hydrogen in the firing furnace or to prevent hydrogen from being absorbed into the steel sheet or to entrap the absorbed hydrogen within the steel sheet.
Various studies have been made from the aspects as mentioned above, and enamelling steel sheets, particularly cold rolled steel sheets, having excellent resistance to occurrence of fish-scales have been developed. Most of these conventional enamelling steel sheets are produced utilizing non-metallic inclusions, such as oxides and sulfides in the steel, which cause formation of a large number of small voids in the steel during the cold rolling, thereby increasing the hydrogen absorbing capacity of the steel sheet. These conventional arts have been found to be very effective with respect to cold rolled steel sheets, but quite less effective with respect of hot rolled steel sheets, and no practically useful hot rolled enamelling steel sheets not susceptible to the occurrence of fish-scales have been realized.
Recently an improved enamelling steel sheet has been developed as disclosed in Japanese Patent Publication Sho 54-39808 in which restricted amounts of boron and oxygen are added to form fine boron oxides so as to increase the hydrogen absorbing capacity. However, it has been found that it is very difficult in a commercial production by this prior art to assure a desired relation between the restricted contents of boron and oxygen, and that this prior art cannot provide satisfactory hot rolled enamelling steel sheet.
Therefore, under the present status of art up to now, although the cold rolled enamelling steel sheets can be nowadays enamelled on both sides satisfactorily, most of the hot rolled enamelling steel sheets are enamelled only on one side and it is very difficult to enamel the hot rolled enamelling steel sheets on both sides.